The main question, in my opinion, is : "if two opposed people have equal skills, and conditions do not favour anyone, what are the chances one of them wins" ?
My personal answer to that is 50%.
If you ignore partial and near successes, and use a binary outcome (100 or less = failure, 101+ = success), then the skill roll must provide equal chances for both outcomes.
- either you do an opposed roll (which is good for PC vs PC, less so for PC vs NPC because the GM has enough things to do already)
- or you adjust the active skill check so that the probabilities of success are 50%. In this case, it means that the roll is made as follows :
roll = <active PC skill> + d100OE - (<opposing NPC skill> -50) [or roll = <active PC skill> +50 +d100OE - <opposing NPC skill>, which is the same thing].
Add situational modifiers as needed, but only add those that have an impact on one skill but not the other (if a conditional modifier impacts both skills the same way, the net result is 0).
If the NPC is active and the PC is reacting, make the PC roll anyway.